Communication systems generally utilize modulation techniques to encode data onto a waveform. Communication systems can include wireless communication systems, cellular communication systems, the global system for mobile communications (GSM), optical networks, local area networks (LANs), wide area networks (WANs), telecommunication networks and other networks. Modulation techniques allow intelligence (e.g., data or information) to be encoded on a carrier wave. Low frequency to very high frequency carrier waves (over several gigahertz (GHz)) can be modulated to provide data or information.
Various modulation techniques have been utilized including digital or analog schemes. Some conventional forms of modulation include, amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), frequency shift keying (FSK), quadrature amplitude modulation (QAM), phase shift keying (PSK), quadrature phase shift keying (QPSK) and hybrids/combinations thereof. Other modulation techniques include minimum shift keying (MSK), binary phase shift keying (BPSK), 16-QAM, code division multiple access (CDMA), time division multiple access (TDMA), etc. The above-mentioned techniques generally adjust the frequency, phase, or amplitude of the carrier wave to encode data onto the carrier wave.
Conventional modulators adjust or generate characteristics of a carrier wave that are varied or selected in accordance with modulating data or information. For example, a signal synthesizer can generate arbitrary modulated waveforms in accordance with digital and/or analog modulation techniques. The modulator can vary a property of an electromagnetic wave or signal, such as its amplitude, frequency or phase, in response to digital data or an analog signal.
Certain conventional modulation schemes, such as quadrature modulation (OM), simultaneously modulate a carrier wave in accordance with two distinct bits of data or streams of information. Quadrature modulation digitally encodes multiple data streams independently The two digitally encoded data streams are referred to as an in-phase (I) signal associated with I data and a quadrature (Q) phase signal associated with Q data. The I and Q data can be generated digitally. The combination of the I and Q signal results in a unique two-dimensional signal vector or symbol.
Quadrature modulation conventionally requires that the I signal and Q signal be generated at base band frequency. The base band frequency is up-converted to higher frequencies, such as, radio frequencies (RF) using a mixer. Quadrature modulation requires significant power and requires large discrete filters to remove unwanted mixer up-conversion products. These disadvantages are particularly problematic in mobile wireless communication systems and other applications with small size and low power design requirements.
Conventional wireless systems have utilized an envelope elimination and restoration (EER) technique to amplify an already modulated signal. The EER technique separates a phase modulated signal and an amplitude modulated signal from the already modulated high frequency signal. After separation, the phase modulated signal and the amplitude modulated signal are amplified in distinct circuit paths and recombined after amplification.
The envelope elimination and restoration scheme allows more efficient amplification circuits to be employed. Although the system allows more efficient amplification, conventional systems have not been able to use this technique in modulation schemes because generating precise phase modulation is difficult and applying phase and amplitude modulation in synchronism is difficult. Inaccurate timing results inaccurate data being encoded on the carrier signal. An EER technique is discussed in McFarland, “An IC for Linearizing RF Power Amplifiers Using Envelope Elimination and Restoration,” IEEE Journal of Solid State Circuits, Vol. 33, No. 12, (December 1998).
Thus, there is a need for a modulation technique which can efficiently modulate data. Further still, there is a need for a modulation technique that can simultaneously amplitude and phase modulate information onto a carrier wave without requiring significant power or filtering requirements. Yet further, there is a need for an apparatus for and a method of modulating a signal efficiently. Further still, there is a need for a method of and apparatus for generating arbitrary modulated waveforms using phase or frequency modulation and amplitude modulation simultaneously.